I. Field of the Invention
This invention relates to a semiconductor device having improved electron mobility, and in particular, to a transistor constructed of GaN/Al.sub.x Ga.sub.1-x N heterostructures that is capable of operating at temperatures greater than 100.degree. C.
II. Description of Related Technology
Al.sub.x Ga.sub.1-x N is a 3-5 semiconductor system with a direct bandgap ranging from 360 nanometers at x=0 to 200 nanometers at x=1. As reported by M. Gershenzon, D. E. Wang and L. Ta, in the proceedings of the 1981 International Opto Electronic's Workshop, National Chang Kung University, Tinan Tai Wan, Page 55 (December 1981), the Al.sub.x Ga.sub.1-x N material has potential uses in ultraviolet light emitting diodes and detectors, short wave length lasers and transit time limited microwave power amplifiers.
Highly uniform epitaxial films of the materials have been grown by reactive molecular beam epitaxy (as reported by S. Yoshida, S. Gonda, and S. Misawa, and the Journal of Applied Physics, Vol. 53, Page 6,844, 1982), as well as by vapor phase epitaxy (as reported by M. Ilegems, and H. C. Montgomery, Journal of Physical Chemistry Solids, Vol. 34, Page 885, 1973) and by atmospheric and low pressure metal organic chemical vapor deposition (See M. A. Khan, R. A. Skogman, R. G. Schulze, and M. Gershenzon, Applied Physics Letters, Vol. 42, Page 430, 1983 and H. Amano, T. Asahi, and I. Akasaki, Japanese Journal of Applied Physics Letters, Vol. 29, Page L205, 1990).
However, these materials are generally unsuited for semiconductor device applications, because the materials produced are highly degenerate N-type (N.sub.d =10.sup.18 to 10.sup.20 per cubic centimeter) and the carrier mobilities are relatively low, typically less than 100 cm.sup.2 per volt second. Researchers generally concluded that the undesirably high carrier densities were caused by nitrogen vacancies formed during the material growth process.
High electron mobility to transistor have been produced in the past from a GaAs/Al.sub.x Ga.sub.1-x As material as reported by S. Hiyamizu, T. Mimura, T. Fuji and K. Nanbu, Applied Physics Letters, Vol. 37, Page 805, 1980. Although this material exhibits superior electron mobilities, it suffers from instability at high temperatures.